The present invention relates to a method for manufacturing parts, with intimate connection of least two material phases, of which at least one is a solid body phase, and in which method, prior to intimate connection, at least the surface of the solid body phase to be connected is pretreated, uses a workpiece stored in air and a vacuum processing chamber comprising a plasma discharge section, a gas supply operatively connected with a gas tank arrangement, and a workpiece holding device.
The term xe2x80x9cintimate connecting methodxe2x80x9d is understood to include bonding (welding, soldering), gluing, potting, and coating, especially in a vacuum coating process, whether PVD or CVD or methods derived therefrom, as well as so-called xe2x80x9cdirect bonding.xe2x80x9d In such a method, carefully cleaned surfaces are connected to one another directly by interatomic forces, as used, for example, in direct wafer bonding of silicon on silicon, silicon on SiO2, and also in the joining of metal surfaces with one another or of metal surfaces with silicon surfaces, for example CuSi or Auxe2x80x94Si. At least one of the surfaces to be joined intimately is always the surface of a solid body.
The following are addressed, in particular, within the framework of the present application:
(a) metal surfaces, especially made of Cu, Ni, Ag, Au Pd, Sn, Ti, In, or alloys containing at least one of these metals;
(b) semi-metallic surfaces, especially made of silicon, germanium, boron, carbon, gallium arsenide, gallium nitride, silicon carbide, zinc oxide, or of a material with at least one of these semi-metals;
(c) ceramic surfaces, especially made of quartz, aluminum oxide, aluminum nitride, zirconium oxide, boron nitride, diamond, and silicon nitride;
(d) plastic surfaces on an epoxy or ester basis, polyimides, polyvinyl chlorides, polyethylene, polystyrene, polyolymethacrylate, polyamide, polyurethane, phenoplasts, phenol resins, silokanes, Teflon; and
(e) in particular, metals like those that are typically used in packaging processes for semiconductors, namely cured epoxy resins, HLST materials (semiconductor system substrates) made of epoxy (epoxy, base laminate substrates), solder resist, photo-resist, etc.
It is important that those surfaces which undergo the intimate connecting method can consist of portions different from the various materials named. The energy supply for the intimate connection inmost cases is of a thermal nature and is supplied for example by heated tools, Joule heat, UV radiation, or preferably by ultrasound to the surfaces to be joined, or by reaction energy during gluing and possibly also during potting.
The present invention in is used especially advantageously in packaging integrated circuits. In this sense, its primary area of application lies in the field of the semiconductor industry. The present invention can, however, also be used for other areas of technology, basically in all those in which, within the framework of the manufacturing method according to the invention, before the creation of an intimate connection, organic or organic/oxidic contaminating compounds must be removed from the (at least one) surface of the solid body.
Although surface materials have been defined above for which the present invention is known to be suitable, it should be noted that other surface materials, for example oxides, nitrides, carbides, oxynitrides, oxycarbides, carbonitrides, and oxycarbonitrides of at least one of the metals Ti, Ta, Zr, and Hr can be processed according to the invention, especially if the intimate connection by the coating is taken into account.
As mentioned above, the procedure according to the present invention is also especially suitable for the surfaces of solid bodies that consist of different materials, especially areas that contain different materials. As far as intimate connection is concerned, such multi-material surfaces pose especially difficult problems.
In the xe2x80x9cpackagingxe2x80x9d of integrated circuits, as a preferred area of application of the present invention, a distinction is made between a plurality of work steps that include an intimate connection of surfaces in the above sense:
Step 1. The individual integrated circuits are cut from a silicon wafer, mounted on semiconductor system carriers (HLST) and joined with them (so-called xe2x80x9cdie-bondingxe2x80x9d). The HLST surface to be joined is usually made of copper or nickel, silver, or gold, and/or from a material with an epoxy basis, generally from a plastic as mentioned above. Examples of such HLSTs are stamped or etched metal lead frames, ceramic substrates, or BGA (ball grid arrays)xe2x80x94substrate carriers made of plastic or printed circuit boards. The connecting methods used include hard soldering, soft soldering, and gluing. In flip chip soldering processes, the integrated circuit is mounted on an HLST by geometrically separate solder balls which are simultaneously used as I/O contacts.
Step 2. Joining the integrated circuits with contact supporting points on the HLST, for example on the xe2x80x9clead frames:xe2x80x9d The surfaces involved are metallic, made for example of Al, Au, Cu, Ni, Pd, Ag, Pb, Sn or alloys of these metals. In this case, soldering or welding are primarily used as joining techniques, especially fluxless soldering or ultrasonic welding. This step is known as xe2x80x9cwire bondingxe2x80x9d.
Step 3. Molding: In this method step, the circuits on the HLST, for example the lead frames, following xe2x80x9cwire bondingxe2x80x9d are potted with a molding compound, with the above-mentioned surfaces of the HLST and the integrated circuits being involved with the molding compound.
A cleaning method is known from EP-0 371 693 within the framework of a manufacturing method in which the surfaces later to be connected in the manner described above exclusively by applying energy are first exposed to a microwave plasma discharge in a vacuum atmosphere containing hydrogen. Then, without breaking the vacuum, the solder layer provided for connecting the surfaces is melted by the plasma discharge. Thus, a contaminated surface coating that would seriously interfere with subsequent joining processes is prevented simply by avoiding contact with air.
It is also known from U.S. Pat. No. 5,409,543 to use activated hydrogen to prepare for a soldering process. As a result, an oxide layer is singled out for the performance of the soldering process on the metal surface.
It is also known from EP-A-0 427 020 to pickle a passive and oxide layer of assembly partners with a high-frequency plasma pre-treatment with a processing gas, in other words to separate it out. The processing gases that can be used include the following gases and mixtures of the gases among others: O2, H2, Cl2, N2O, N2, CF4, etc. The above-mentioned pickling does not take place immediately before the soldering process as in U.S. Pat. No. 5,409,543, so that the members to be joined are stored in protective intermediate storage locations, for which purpose suitable containers under a protective gas are provided to prevent contamination.
Within the framework of the three packaging steps mentioned above, two plasma processing steps have been introduced in the meanwhile. They are intended primarily for substrate carrier materials made of plastic (xe2x80x9cplastic ball grid arraysxe2x80x94PBGAxe2x80x9d). The plasma processing before the xe2x80x9cwire bondingxe2x80x9d (Step 2) serves to clean the surfaces of the metal contact support points (xe2x80x9ccontact padsxe2x80x9d) usually made of aluminum, on the die (the chip) and the contact support points that are usually made of gold on the system substrate (HLST), so that electrical contact with the connecting wire, usually gold wire, is ensured. The most important source of contamination of these contact support pads is a previous treatment process (xe2x80x9ccuringxe2x80x9d) that is used for curing an epoxy adhesive used in the Step 1.
A second plasma treatment usually takes place only after the xe2x80x9cwire bondingxe2x80x9d (Step 2), prior to the xe2x80x9cmoldingxe2x80x9d process step (Step 3). This is intended to achieve an improved adhesion of the molding material.
Both the plasma processing steps mentioned above were usually performed in a vacuum under plasma gas excitation. A high-frequency, microwave, or ECR plasma is usually employed. The plasma surface interaction and the cleaning process associated with it or the surface modification takes place by sputtering and/or a chemical reaction with plasma-excited gases. For sputtering in a non-active atmosphere, usually it only argon is used and the self-bias potential of the substrate to be cleaned and operated floating is utilized to accelerate argon ions toward the latter and to achieve the desired removal of the cleaning material. In plasma-chemical removal, reactive gases, for example oxygen, are excited, dissociated, or ionized and then enter into a reaction with surface impurities, for example carbon, and carry away the gaseous reaction products such as CO2 through the pumping system.
As in the present case, a highly advantageous processing method has become, known from WO 97/39472, by using it as the above-mentioned plasma processing method. In this process, hydrogen is excited preferably in a plasma discharge and then removes carbon from surfaces (for example as CH4) while simultaneously reducing oxides on the surfaces involved, to produce H2O gas. Sputtering is avoided, which involves the risk of redeposition, and the above-mentioned process can be applied without limit to metallic substrate carrier materials or silver contact surfaces as well, that would oxidize severely in an atmosphere containing the excited oxygen, especially in a plasma containing oxygen. This would lead to an adverse effect on xe2x80x9cwire bondabilityxe2x80x9d or xe2x80x9cmoldabilityxe2x80x9d (see above, Steps 2 and 3). This is also particularly important if applications for the modern copper metallization of chips is being considered. One very important property of this process is that the surfaces are passivated for a technologically sufficient period of time by the hydrogen plasma and so can be stored in air before entering into an intimate connection in the above sense.
A disadvantage of the known process is that the process window, in other words the range of process parameters still depends to a certain degree on the material of the surfaces to be processed and to be joined intimately later. Thus, for example, the plasma processing of substrate carrier materials (strips) normally takes place in magazines. These magazines have slits so that the gases excited in the plasma can penetrate to reach the substrate surfaces to be treated in the magazine and the volatile reaction products of easily can leave the magazine cleaning and be pumped off. The substrate surfaces, especially in the case of PBGA (xe2x80x9cplastic ball grid arrayxe2x80x9d) strips, always consist of a plurality of different materials. The xe2x80x9csolder resistxe2x80x9d is a long-chain organic compound, the surface of the xe2x80x9cdiexe2x80x9d (chip) consists for example of polyimide or silicon nitride, the metallization part usually consists of aluminum and gold.
It can never be ruled out that additional organic impurities remain on the surface of the xe2x80x9csolder resistxe2x80x9d that originate on strips from previous processes, e.g. cleaning steps, are precipitated during the curing of the epoxy in the xe2x80x9ccuring processxe2x80x9d, or result from the deliberate surface treatments on the xe2x80x9csolder resist,xe2x80x9d treatments to promote for example the wettability with the molding material. Highly volatile compounds may also be present that evaporate in particular at the beginning of the above-mentioned magazines, generally in small spaces, resulting in locally high pressures. It can happen that the chemical equilibrium is altered by these by the local pressure shifts so that no more hydrocarbons can be removed from the reactive hydrogen in the above-mentioned process, but rather polymerization is promoted. The result of such re-covering can be extremely diverse. Thus, for example, at the contact support points (xe2x80x9ccontact padsxe2x80x9d) the xe2x80x9cwire bondabilityxe2x80x9d relative to the pull strength achieved is reduced, or much longer processing times are required in order to remove the deposited layer again at optimum pressure and possibly at higher temperature.
Because of the above-mentioned diversity of materials on the surface to be treated, however, a prolonged processing time or a more intense plasma processing can for example treat some surfaces such as metal surfaces, better while at the same time, others, such as the xe2x80x9csolder resistxe2x80x9d or the passivation layer can be changed so that the adhesion of the molding material to be added later is made worse (see the discussion of Tests II).
An object of the present invention is to provide a method for manufacturing parts in which the process window, especially the pre-treatment processing window and its dependence on various surface materials and also on process parameters such as xe2x80x9cpressurexe2x80x9d and xe2x80x9ctemperature,xe2x80x9d is widened. In other words, the method proposed according to the present invention, under expanded parameter ranges, such as pressure ranges and/or temperature ranges, is intended to provide uniformly satisfactory results over the entire variety of materials mentioned, as far as the quality of intimate connection to be provided later is concerned.
On the basis of this enlarged process window, an improved homogenization of the processing effect distribution is also to be ensured on multi-material surfaces.
As mentioned above, in particular, the results should also be ensured for substrates in magazines or for substrates with narrow geometries (slits and holes). Since the plasma-chemical reactions on the substrate surfaces also depend on the substrate temperature and larger temperature gradients are involved, especially for plasma processing in magazines and on extensive substrates, the above-mentioned process window enlargement is also intended to promote uniformity.
Thus, a constant surface treatment, including gap surfaces, hole surfaces, groove surfaces, etc. is to be made possible on substrates themselves.
In addition to the re-covering of the treated surface by deposition or polymerization being prevented, the method of the present invention is economical, and no explosives and/or environmentally hazardous gases need be used. The advantages of the above-mentioned known methods, such as the one described in WO 97/39472, especially its preservative properties are maintained.
The following additional definition applies: passivation or passivized; see Rxc3x6mpps Chemielexikon, Franksche Vrelagshandlung, Stuttgart, 9th Edition, Page 3005. This term refers to a bonded protective coating on the surface of the solid. The clean solid body surface is protected against atmospheric air influences. This is accomplished by, for example, forming an oxide or nitride layer. Such a layer must first be broken up by energy applied specifically for this purpose for producing an intimate connection of the type described above, for example by application of higher temperatures than required for the actual joining process, or chemically, for example by using a flux.
A basic distinction exists between the above-mentioned xe2x80x9cpassivationxe2x80x9d and preservation which in particular does not require any layer separation by additional energy for joining. This preservation is known in conjunction with the method in WO 97/39472, the disclosure of which is incorporated herein by reference.
The above object has been achieved by a method in which the pre-treatment is performed using plasma-activated gas that contains nitrogen. In addition, in a currently preferred embodiment of a manufacturing method according to the invention, the plasma-activated gas also contains hydrogen.
Although it is quite possible to use any one of the many known plasma discharge types for plasma activation of the above-mentioned gases, in a currently preferred embodiment the plasma discharge is produced as a low-voltage discharge, preferably with a thermionic cathode. In addition, the plasma-activated gas preferably contains a working gas, preferably a noble gas, especially argon.
The above-mentioned plasma-activated gas, in additional to nitrogen, contain certain gas components, especially hydrogen and/or a working gas, can contain in a preferred manner primarily nitrogen, and can even consist of nitrogen apart from any working gas that might be provided.
The solid body surfaces to be joined intimately are materials that are metallic and/or semi-metallic and/or ceramic and/or consist of plastic, especially according to the materials mentioned at the outset and deemed currently preferred. Preferentially, the above-mentioned solid book solid body surfaces are surfaces with areas composed of different materials, especially the materials mentioned.
Th intimate connection is formed by the manufacturing method according to the present invention, preferably by gluing, soldering, welding, molding, or coating, especially vacuum coating, performed by so-called xe2x80x9cdirect bonding.xe2x80x9d
The above-mentioned, preferentially-used low-voltage discharge is also operated preferentially with a discharge voltage of 30 V or more, preferably with a discharge current between 10 A and 300 A, including both limits, especially at 40 A to 70 A.
In a currently preferred embodiment of the method according to the invention, the at least one solid body surface to be connected intimately later, following the above-mentioned pretreatment and prior to its intimate connection, is exposed to air, for periods ranging from days to weeks. Thus, it is no longer necessary to further process the above-mentioned surface immediately after its pretreatment and at the same location. The result is a high degree of flexibility regarding xe2x80x9crhythmxe2x80x9d and processing location for working the method according to the invention without requiring additional costly cleanliness precautions such as storage under a protective gas to be provided.
In an especially preferred embodiment of the method according to the present invention, at least the one solid body phase is stored in a holder during pre-treatment. The holder defines access areas on the above-mentioned surface that are narrowed relative to the plasma discharge chamber. In the overwhelming majority of cases, the at least one solid body phase is formed by a disk- or plate-shaped substrate, and the holding device is provided with at least one access slit. Preferably, the holding system has several of the above-mentioned access slits and forms an actual magazine.
One preferred use of the method according to the present invention consists in joining integrated circuits with HLST or in the electrical connection of integrated circuits by xe2x80x9cwire bonding,xe2x80x9d or for sheathing electrical circuits connected with HLST and connected by xe2x80x9cwire bondingxe2x80x9d with a molding material.
Another preferred use of the method according to the present invention is for integrated circuits for flip chip connection and for positioning. Firstly, the soldering points can be cleaned of oxide and passivated at the same time and secondly, following plasma processing, a better wetting of the so-called xe2x80x9cunderfillxe2x80x9d (a molding material that fills the gap between the chip and the chip carrier and which serves to absorb mechanical stresses) is achieved.
In addition, the method according to the invention is also used preferably for workpieces with surfaces to be connected intimately with poorly accessible surface areas, especially edges, holes, blind holes, gaps, and grooves. The method according to the invention is also especially suitable for chips with copper traces.
The workpiece that has been processed with plasma according to the present invention and stored in air is characterized by the fact that the above-mentioned surface that has been exposed to air has an increased nitrogen concentration by comparison with a directly produced surface of the workpiece which however was not stored in air. This can be demonstrated by, for example, xe2x80x9cFourier transform infrared spectroscopyxe2x80x9d with xe2x80x9cattenuated total reflection,xe2x80x9d FTIS-ATR, and/or with xe2x80x9cnuclear reaction analysis,xe2x80x9d NRA, or with xe2x80x9ctime-of-flight secondary ion mass spectrometry,xe2x80x9d or TOP-SIMS.
As a result of the elevated nitrogen concentration, which indicates,the pre-treatment according to the present invention, the workpiece according to the invention can be used and stored in air directly without additional pretreatment in the above sense for an intimate connection. In yet another currently preferred embodiment of the method according to the invention, the intimate connection can be performed by the influence of heat on air, preferably at a solid body temperature of 150xc2x0 C. at most.
A vacuum processing chamber according to the invention is also characterized by a gas tank arrangement containing a gas with nitrogen and at least one access slit open to the discharge chamber for a disk-shaped or plate-shaped-workpiece.
The influence of nitrogen as recognized and utilized according to the present invention is surprising, in the light of the teaching in the art (see J. L. Vossen et al., xe2x80x9cThin Film Processes,xe2x80x9d Academic Press, Inc. 1978) according to which conventional thinking believed that nitrogen plasmas did not remove polymer surfaces.